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American Kestrel Nest Box Project Year 7: Surprises, successes, and research implications

September 15, 2022 Category: , , ,

Year 7 of the American Kestrel Nest Box Monitoring Project’s season started off with high expectations. Considering the high expectations, the season sort of ended abruptly, without much pomp and circumstance. Towards the beginning of July, which is getting toward the end of our monitoring season, there were four boxes with eggs that we were watching and eagerly waiting on to hatch. Our final banding date of July 15th came and went… and nothing hatched. I was hopeful that the last four boxes that had eggs would hatch, but alas, they never did and so we ended our season for the American Kestrel Nest Box Project on July 15th. That is the way that things go some years.

For such an unsatisfying end to the season, how did we get started this year?

Frank tightening down the final bolt of a new box placed in the Sax-Zim Bog.

April is a month of preparation for the project. Typically, we check our notes about what boxes need repair or replacement, emails get sent to volunteers to check in, and a day to clean out previously used boxes gets scheduled. This year, however, Frank Nicoletti and I, with the help of Jason Heinen spent a couple of days in early April cleaning out and repairing EVERY box that we could get to in the Bog! This means around 50 boxes were cleaned, repaired, moved, adjusted, and maintained. We also added one more box to the landscape, bringing our usable box tally up over 50! We had 51 usable boxes this season, which is the first of the many records seen this season. 

This box was not filled by mice, but a Northern Flicker! Woodpeckers usually are removing material to make a cavity, however, as soon as this box was cleaned a flicker promptly began filling it in!

Following box cleaning, Jason Heinen and I spent a day checking for the return of banded American Kestrels to box locations. If you recall, last year was the first year of the project where nestlings were color banded and a few adult birds got color bands, as well as transmitters. The blue color bands are easily seen with the help of spotting scopes and even seen in photographs. The transmitters mentioned above were placed as part of a project in collaboration with the Minnesota DNR to help monitor kestrels in migration with the use of MOTUS Towers (like the one we have at the Welcome Center!). Not all transmitters pinged back during the fall and spring migrations of American Kestrels in the Bog, so we headed out and checked for the return of these birds at the previous nesting locations. No previously banded birds were seen; however, we did note a lot of interesting pre-breeding behaviors.

We saw a number of male American Kestrels carrying Red-bellied Snakes near nesting locations to give to females as part of courtship. One male was even calling inside of a box location to get a female to come closer so he could give her the snake he had in his talons within the box! Copulation was observed at a number of places and we had a great day looking for and at these lovely raptors. 

After choosing a nest site, female kestrels get comfy. They will make a small depression at the bottom of the cavity or ledge they have chosen and begin to lay eggs.

At the start of each season, I make sure that our volunteers are well prepared and we complete a training for both new and returning volunteers. I am truly grateful to have such an experienced and energetic group of volunteers for a project of this scale! All told, 110.75 hours of volunteer time were put into the monitoring aspect of the project. An unbelievable amount of time! The work done by our volunteers also included a little extra data collection this season.

Halle in action! Here, she is taking morphometric data from American Kestrel nestlings. Every chick from each box in her study is weighed, four different measurements are taken, and a couple of feathers are also taken for later analysis.

For the next couple of seasons our boxes will be included in a research project by University of Minnesota-Duluth Master’s Student Halle Lambeau who is working on American Kestrel nesting in the upper Midwest. Her study includes data and observations from over 450 boxes spanning Minnesota from the Sax-Zim Bog to the Twin Cities and into Central and Southern Wisconsin! There are a number of collaborators with Halle on this project and we are happy to share and contribute data to this project for Halle and American Kestrels! There will be much more about this project to come in the future, so stay tuned.

The final exciting note about this season is that we have finally collected enough data to begin analyzing it for an upcoming paper about our project! With 5 years of robust data over the course of 7 years of this project we will be able to analyze data and give insights on clutch sizes, egg laying, nest timing, success and failure rates, and more. Regional data is significant because birds in different regions do different things at different times, especially if they are migratory species like American Kestrels and these regional data points will allow for more informed conservation methods in the future.

Now… the exciting part… how did the boxes do this year?

An up-close look at this male American Kestrel nestling getting a wing chord measurement.

As noted above, we had a record number of useable boxes this season, with 51 useable boxes on the landscape. And as you might expect, a year with a record number of boxes to use, might just lead to a record number of boxes used! We had 32 boxes occupied by American Kestrels this season, an unprecedented leap over the record 23 occupancies last season. Of those occupied boxes 26 produced chicks (another record!). Two nests were predated this year, with an additional 4 being abandoned. 

This batch of fluffy kestrels have been banded, weighed, measured, and are waiting to be placed back into their box. The whole process takes less than 20 minutes, no matter the clutch size.

A total of 152 eggs were laid and 123 of those eggs were found in successful nests. Of those 123 eggs, 108 hatched. From that total, we banded 105(!) chicks this season, which is of course a record. We also surpassed an amazing milestone: over 300 chicks have now been banded as part of this project! In fact, we are closing in on 400 chicks banded over the last 7 years. Like last year, a few adult kestrels were banded and were fitted with transmitters. This year, an additional 10 adults were fitted with transmitters and will hope to continue to build our understanding of their migration. 

This season also had its fair set of surprises.

Our first surprise came during our box cleaning efforts in early April. We are no longer surprised by mouse nests, or even mice and voles overwintering in boxes. One thing we did not expect to find in a nest box, however, was two dead kestrel nestlings. These nestlings came from a box that had not been occupied during the duration of our monitoring season and was likely occupied too late in the season for a successful nesting effort. The kestrels we found were the first nestling mortalities we have documented as part of the project.

Our second major surprise, unfortunately also regarding mortalities, came from a couple of grizzly observations during banding. If you noticed above, 108 eggs hatched, but we only banded 105 chicks. Well, we found two different instances of siblicide in boxes this year, counting for a loss of 3 chicks. This nestling on nestling predation is not uncommon in larger raptors (Bald Eagles for example), but is not terribly common in American Kestrels. Halle also documented at least one instance of siblicide in the boxes she was monitoring for her project in Douglas County, Wisconsin. This is only the second time we have ever found nestling mortality in our boxes. 

And finally, on a much happier note, we had a banded bird return to a nest site!!!

If you look closely at her left leg, you can see the blue color band! We had previously only colored banded birds during the 2021 season, which means this bird was banded in the Bog and last year!

As noted above, during our spring survey of adult kestrels, we did not find any banded adults returning to nest sites. However, our volunteers did discover at least one banded adult using a nest site this season. A color banded adult female American Kestrels was captured on video laying eggs and occupying Box 8! This is the first banded kestrel we have detected using a nest location in the Sax-Zim Bog. Her observation is significant due to her color band, which we had only began to place on birds last year (on both adults and nestlings). Because of this blue band we can be sure of two things: 1) She was banded in the Sax-Zim Bog! 2) She was either a chick last year OR and adult that had been carrying a transmitter last year. Unfortunately, and unexpectedly, many of the transmitters placed last year failed and detached from a number of birds and we were not able to catch her, so we likely can only guess who that bird was and where she was last season. Lots of questions, but still very, very exciting!

Our final box of kestrels were banded on July 15th. These chicks are around 23-25 days old and nearly ready to leave their box for their first flights at around 28-30 days.

What an amazing season. The Sax-Zim Bog is an absolutely an amazing set of habitats. One that not only supports the boreal species we love, like Boreal Chickadee, Great Gray Owl, Connecticut Warbler, Black-backed Woodpeckers and beyond. But it also supports grassland and open field birds like American Kestrels. If you visited the Bog after the month of July, hopefully you noticed the great numbers of American Kestrels in the Sax-Zim Bog. I appreciate the support from our volunteers and those folks who support the Friends of Sax-Zim Bog so we can continue to support conservation in the greater Sax-Zim Bog!

Halle and Head Naturalist Clinton with the last chicks banded during the 2022 season. (Photo by Frank Nicoletti)

To read more about our American Kestrel Nest Box Project, check out our previous Bog Blog posts here.

—- Head Naturalist Clinton